In recent years, sending and receiving of a high-quality moving image has become popular with the spread of the Internet, and as a result, a volume of information circulating on a communication network gets larger, resulting in a surge in communication traffic.
To cope with such a surge in communication traffic, there has been introduced a transmission system applying a modulation/demodulation method such as differential phase shift keying (DPSK) with a transmission capacity of 40 Gb/s per wavelength in a trunk line. The DPSK is application of PSK that assigns different values to waves which differ in phase from a reference sinusoidal wave thereby transmitting/receiving a signal, and is a technique to detect a phase of a wave on the basis of a phase of the last wave (one-bit preceding wave) transmitted so that a phase can be identified even in the absence of a reference signal.
Furthermore, besides the DPSK, modulation/demodulation methods such as differential quadrature phase shift keying (DQPSK) and dual polarization quadrature phase shift keying (DP-QPSK) are also applied. The DQPSK is application of QPSK that assigns 2-bit data to four modulated phases, respectively, and detects a phase of a carrier wave using a difference from the last carrier wave. The DP-QPSK is a technique to transmit a QPSK signal through two polarized waves. Moreover, at present, DP-QPSK with a higher capacity of 100 Gb/s was standardized by the Optical Internetworking Forum (OIF) and has been developed.
Furthermore, to cope with the surge in communication traffic, there has advanced study of a higher-capacity optical transmission system by the use of an optical modulator applying multi-level optical modulation, such as optical 16 quadrature amplitude modulation (QAM). The QAM is due to a combination of amplitude modulation and phase modulation, and is a modulation/demodulation method for transmitting multiple pieces of information at a time by varying both elements: amplitude and a phase.
The optical modulator modulates, for example, a carrier light emitted from a laser diode (LD) on the basis of a drive signal based on a transmitting signal, thereby generating a modulated optical signal. Here, if the amplitude of the drive signal for driving the optical modulator deviates from appropriate amplitude, distortion of constellation occurs, and this causes signal-noise (S/N) ratio degradation of the modulated optical signal. Therefore, control of drive amplitude so as to be an optimum value is required.
In this regard, in conventional technologies, there is a known technology to control amplitude of a drive signal to be the optimum amplitude in an optical modulator applying DQPSK modulation in such a manner that the optical modulator modulates the drive signal by varying the amplitude minutely with a low-frequency wave and controls the amplitude of the drive signal so that the low-frequency component contained in a modulated optical signal is zero. This takes advantage of characteristics of the optical modulator applying DQPSK modulation—that is, the low-frequency component is contained in a modulated optical signal when the amplitude of the drive signal deviates from an appropriate value; on the other hand, the low-frequency component is not contained in a modulated optical signal when the amplitude of the drive signal is the appropriate value.    Patent document 1: Japanese Laid-open Patent Publication No. 2008-092172
However, the conventional technology does not take into account the control of amplitude of a drive signal appropriately in more multi-level optical modulation, such as 16QAM optical modulation.
Namely, for example, DQPSK modulation is achieved by combining phase modulation of two values which differ in phase by π, such as 0/π. Consequently, when amplitude of a drive signal is an appropriate value, a low-frequency component superimposed on the drive signal in the phase of 0/π is output as a frequency component of a frequency twice as high as the low-frequency component, so no low-frequency component is contained in a modulated optical signal. Therefore, by controlling amplitude of a drive signal so that a low-frequency component contained in a modulated optical signal is zero, the amplitude of the drive signal can be controlled to be the optimum amplitude.
On the other hand, more multi-level optical modulation, such as 16QAM optical modulation, is achieved by combining phase modulation in a phase of 0<φ<π in addition to 0/π. When the conventional technology is directly applied to such an optical modulation, even if amplitude of a drive signal is the appropriate value, a low-frequency component superimposed on the drive signal is output as a frequency component as-is in the phase of 0<φ<π, so the low-frequency component is contained in a modulated optical signal. Namely, regardless of whether the amplitude of the drive signal is optimum, the low-frequency component is contained in the modulated optical signal; therefore, it is difficult to determine whether the amplitude of the drive signal is optimum. If the amplitude of the drive signal is controlled so that the low-frequency component contained in the modulated optical signal is zero, it may be difficult to control the amplitude of the drive signal to be the optimum amplitude.